In general, in an internal combustion engine equipped with a supercharger, a cooling control system including an intercooler cools intake air which has been increased in temperature by being pressurized by the supercharger, in order to avoid occurrence of knocking or the like while ensuring improvement of power output. Conventionally, as such a cooling control system, there has been known one disclosed e.g. in PTL 1. This cooling control system includes an engine cooling circuit which is provided for mainly cooling a body of the engine (hereinafter referred to as the “engine main unit”) and through which flows relatively high temperature coolant (hereinafter referred to as the “high-temperature system coolant”), and an intercooler cooling circuit which is provided for mainly cooling an intercooler and through which flows relatively low temperature coolant (hereinafter referred to as the “low-temperature system coolant”).
The engine cooling circuit includes the above-mentioned engine main unit, a high-temperature system radiator, and a coolant passage connecting the engine main unit and the high-temperature system radiator, and is configured such that the high-temperature system coolant is delivered by a pump of a mechanical type (hereinafter referred to as the “mechanical pump”) driven by the engine main unit, and is thereby caused to circulate through the engine cooling circuit. On the other hand, the intercooler cooling circuit includes the intercooler, a low-temperature system radiator, and a coolant passage connecting the intercooler and the low-temperature system radiator, and is configured such that the low-temperature system coolant is delivered by a pump of an electric type (hereinafter referred to as the “electric pump”), and is thereby caused to circulate through the intercooler cooling circuit.
Further, the respective coolant passages of the above engine cooling circuit and intercooler cooling circuit are connected to each other at two locations, and the connection passages of them are each provided with an openable and closable valve capable of controlling the degree of opening thereof. More specifically, a first connection passage is connected between downstream of the mechanical pump and the engine main unit and also upstream of the high-temperature system radiator of the engine cooling circuit, and downstream of the low-temperature system radiator and also upstream of the electric pump of the intercooler cooling circuit, and the first connection passage is provided with a first valve. Further, a second connection passage is connected between downstream of the high-temperature system radiator and also upstream of the mechanical pump of the engine cooling circuit, and downstream of the electric pump and the intercooler and also upstream of the low-temperature system radiator of the intercooler cooling circuit, and the second connection passage is provided with a second valve.
Further, the above-mentioned engine is equipped with an EGR device for recirculating part of exhaust gases discharged into an exhaust passage (hereinafter referred to as the “EGR gases”) to an upstream side of a compressor of the supercharger in the intake passage. Therefore, when EGR is being performed, the intake air and the EGR gases (hereinafter, collectively referred to as the “intake gases”) are cooled via the intercooler after being raised in temperature by the compressor of the supercharger, and are introduced into cylinders of the engine in a state lowered in temperature.
The EGR gases usually includes a relatively large amount of water vapor, and hence if the intake gases are excessively cooled by the intercooler, the water vapor in the intake gases is condensed when the intake gases pass though the intercooler, whereby condensed water is sometimes generated in the intake passage. If such condensed water is attached to components of an intake system including the intercooler, the components can be corroded. To avoid this problem, in the above-described cooling control system, the temperature of the intake gases at an outlet of the intercooler (hereinafter referred to as the “outlet temperature”), that is, the temperature of the intake gases cooled by the intercooler is compared with the dew-point temperature at that time, and the outlet temperature is controlled such that it becomes higher than the dew-point temperature.
More specifically, when the outlet temperature is higher than the dew-point temperature, the electric pump is operated with each of the first and second valves in a closed state to cause the low-temperature system coolant to circulate through the intercooler cooling circuit, whereby the temperature of the intake gases is lowered. On the other hand, when the outlet temperature is not higher than the dew-point temperature, the electric pump is stopped, and in addition, the high-temperature system coolant in the engine cooling circuit is caused to flow into the intercooler circuit by opening the first and second valves, whereby the temperature of the low-temperature system coolant is raised. Through this operation, the outlet temperature is made higher than the dew-point temperature, to thereby suppress condensed water from being generated when intake gases are cooled by the intercooler.